Unfortunately, one consequence of replacing a wired connection with an opto-coupled connection for the purpose of isolation is that the opto device consumes power.
Next generation AC/DC laptop adapters need to meet increasingly stringent limits on standby power dissipation mandated by the European Commission's EcoDesign Directive, Energy Star specifications and others and also commercial specifications set by equipment manufacturers. Currently, designs meeting a 30 mW standby power limit are being proposed.
Achieving such low standby power dissipation requires that every step be taken to reduce un-necessary power losses. This includes steps to reduce for example, bias power, voltage sensing losses and control feedback system losses. The general architecture of these AC/DC adapters is that there are two power conversion stages. The first stage acts to control the adapters Power Factor and the second stage acts to regulate and isolate the output. In a typical design, the second stage consists of control circuit and a switch to control the connection of supply from the first stage to the second stage for output regulation as required by the load demand. A feedback signal (FB) of some kind is needed to allow the control circuit to sense the state of the output. This signal may be linear or hysteretic in nature. A linear FB signal is proportional, or inversely proportional to the output, a hysteretic FB signal has two states indicating a Switching/No Switching demand to the controller.
The controller then operates the power stage switch appropriately. Unfortunately, the generation of the FB signal is typically most lossy when the output is above the desired control setpoint, usually associated with operation at light load or no load. This is primarily due to the fact that the operating sense of the opto-coupler used to transfer the FB signal across the necessary isolation barrier is that, in the case of a linear FB signal, the current in the opto-coupler LED increases to its maximum when the output Voltage (Vo) is greater than the required regulation setpoint (Vset). In the case of a Hysteretic FB signal, the current in the opto-coupler LED is turned ON when the output Voltage (Vo) is greater than the required regulation setpoint (Vset). The net effect in both cases is that maximum LED current occurs at minimum load.
Obviously, current is needed to turn the LED on. The associated power loss would typically be 28 mW (for example 1.5 mA or so taken from a 19V output through an appropriate bias resistor) or almost 95% of the total standby power budget for a new design.
The opto-coupler configuration arises because when the output voltage is zero, as in a start-up event, the required state of the opto-coupler LED is OFF, which is the only available LED state because there is no power source available to set the LED ON. The default operation of the controller is then to initiate switching to increase the output voltage which is exactly what is needed to bring the system into regulation.
In order that features and advantages of the present invention may be further appreciated some embodiments will be described by way of example only and with reference to the accompanying diagrammatic drawings, of which: